LATE JURASSIC STRATIGRAPHY, SEDIMENTOLOGY, AND GEOCHRONOLOGY OF THE ALTAR-CUCURPE BASIN IN THE VICINITY OF CUCURPE, SONORA, NORTHWESTERN MEXICO: IMPLICATIONS FOR MESOZOIC TECTONICS OF THE SOUTHWESTERN NORTH AMERICAN MARGIN

The Upper Jurassic stratigraphy of the Altar-Cucurpe basin provides new insights into the tectonic regime of the southwestern margin of North America during the Mesozoic. Upper Jurassic strata of the Altar-Cucurpe basin consist of the Cucurpe Formation of Oxfordian to Tithonian age overlain by the La Colgada Formation of Tithonian age. The Cucurpe Formation consists of ~800-1000 meters of black to medium gray marine shale, turbidite, tuffaceous sandstone and siltstone containing subaqueously erupted pumice, and uncommon beds of siliceous ash fall tuff. Petrographic analysis indicates the Middle Jurassic volcanic arc as the primary sediment source. The Cucurpe Formation is interpreted to represent stacked channel and levee/overbank slope deposits deposited within a rapidly subsiding marine basin that experienced episodic volcanism. Facies relationships and paleocurrent indicators suggest an overall south-southeastern sediment transport direction. An early Kimmeridgian ammonite discovered ~12 kilometers northwest of Cucurpe within the Cucurpe Formation confirms the presence of middle Upper Jurassic strata, which have not yet been identified at the type locality near Rancho La Colgada. The ~200 meter thick La Colgada formation conformably overlies the Cucurpe Formation. It includes shallow marine greenish grey siltstone containing calcite nodules and less common fluvial/deltaic conglomerate and sandstone intercalated with uncommon siliceous tuff. This Unit is interpreted to represent the Tithonian filling of this rift basin. Two tuffs from the La Colgada Formation have been dated using U-Pb SHRIMP analysis of zircon, producing ages of 152.1 +/- 1.7 Ma and 150.6 +/- 1.2 Ma. Although the contact of the La Colgada and overlying Morita Formation of Early Cretaceous age appears concordant, sandstone petrography indicates an abrupt shift in composition, possibly due to a period of nondeposition and/or erosion before the onset of Morita deposition. Geochronology of the Lower Morita Formation will aid in resolving this issue. Evidence for intrabasinal volcanism suggests that Late Jurassic extension took place in an intra arc or back arc setting, probably during westward migration of the Cordilleran arc.